1. Field of the Invention
This invention relates to a grinding disc dressing apparatus for use in a surface grinding machine in which one side (active grinding surface) of a grinding disc acts to grind a workpiece. Such a dressing apparatus is best suited for use in a double spindle-head grinding machine.
2. Prior Art
To achieve high-precision, high-efficiency grinding, there has been proposed a grinding machine employing a diamond grinding wheel or a grinding wheel containing super abrasive grains made, for example, of cubic boron nitride. Such a grinding machine has excellent grinding performance. However, such super abrasive grains are very hard, and therefore when the super abrasive grain grinding wheel is dressed by a diamond tool comprising a single point diamond, the diamond is subjected to premature wear. Thus, such a diamond tool is not suited for dressing the super abrasive grain grinding wheel. For this reason, it is a common practice in the art to dress such a grinding wheel using a cylindrical rotary dressing diamond roll having diamonds embedded in the outer peripheral surface of the roll.
In such a grinding wheel dressing apparatus with a rotary dresser for use in a grinding machine in which the grinding operation is effected by an outer periphery of a cylindrical grinding stone, a shaft which is parallel to the axis of the grinding stone and has a rotary dressing roll fixedly mounted thereon is driven for rotation together with the rotary dressing roll so as to dress the grinding wheel. In the dressing of such a grinding stone having the active grinding surface on its outer periphery, the rotary dressing roll is rotated at a constant speed, and at the same time is fed to the same direction of grinding wheel axis at a constant rate relative to the grinding stone. Since the grinding stone is rotated at a constant speed, the amount of feed of the rotary dressing roll per revolution of the grinding stone is constant. The ratio of the peripheral speed of the grinding stone to the peripheral speed of the rotary dressing roll is constant, and therefore the required conditions of the dressing can be easily satisfied.
With respect to such a grinding stone dressing apparatus, a sufficient space is available around the outer periphery of the grinding stone, so that the grinding stone dressing apparatus can be readily mounted in such a space, and the drive device for rotating the rotary dressing diamond roll as well as the feed device can be mounted without undue difficulty.
However, in a grinding wheel dressing apparatus for use in a surface grinding machine of the type in which a grinding stone or wheel (generally called a grinding disc or an abrasive disc) has an active grinding surface at one side thereof, the rotary dressing roll is required to be fed from the outer periphery of the grinding disc to the central portion thereof. For this reason, the grinding disc dressing apparatus inevitably becomes complicated in construction, and besides it is rather difficult to satisfy the required conditions of the dressing. Here, let's assume that the rotary dressing roll is caused to act on the active grinding surface of the grinding disc and is fed either from the outer periphery to inner periphery of the grinding disc or the inner periphery to outer periphery. When the grinding disc is rotated at a constant speed, the amount of feed of the rotary dressing roll per revolution of the grinding disc can be made constant by feeding the rotary dressing roll at a constant speed radially of the grinding disc. However, if the rotary dressing roll is rotated at a constant speed as is the case with the grinding machine having the cylindrical grinding stone whose outer periphery serves as the active grinding disc, the ratio of the peripheral speed of the rotary dressing roll to the peripheral speed of the grinding disc at the operating point of the rotary dressing roll acting on the grinding disc is not constant. Therefore, the dressing by the rotary dressing roll on the outer peripheral portion of the grinding disc differs from the dressing on the inner peripheral portion of the grinding disc which has a lower peripheral speed.
Further, the opposed sides (active grinding surfaces) of a pair of grinding discs used in a double spindle-head grinding machine can be properly dressed by using a rotary diamond roll. However, sufficient space is not available for satisfactorily accommodating the dressing apparatus, and it is rather difficult to mount the drive device for rotating the rotary dressing diamond roll and the feed device in such a manner that the two devices can satisfactorily perform the grinding disc dressing operation.